Add the connector contract, reusable nodes and per-port intervals

Connectors are the device-facing node class third parties write, so the
surface they build against is versioned and documented: ConnectorNode carries
a declared contract version, a polling loop that publishes only what changed
and reports health around it, and parameters whose credential fields are
marked x-secret so the editor offers the secrets store instead of a text box.
They are found through the fluksio.node_types entry point group, with the
package's own metadata as the manifest. docs/connectors/ has the contract and
the authoring guide; connector-skeleton/ is a working one to copy.

The controller no longer knows what any node type is: start, stop and
report_health are protocol methods on Node, and the built-ins were migrated to
them first, so the hooks a connector implements are the ones the engine has
been driving all along.

Marking a node reusable moves its source to _lib/ and points the node at it by
name. Other flows instantiate it with their own ports and settings, one fix
reaches all of them, and a shared source still in use cannot be deleted.

Ports gained an interval: an output publishes, and an input wakes its node, at
most every n seconds. State keeps the latest value, so only the delivery is
skipped, and pressing Run is never throttled.

Also fixes autosave sending no version on its first save of a session, which
made every flow saved more than once conflict with itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/Flow/FlowEditor.tsx

@@ -453,8 +453,8 @@ function FlowEditorInner({
   })
 
   const addNode = useCallback(
-    (type: string) => {
-      const id = uniqueNodeId(definitions, type)
+    (type: string, sourceRef?: string) => {
+      const id = uniqueNodeId(definitions, sourceRef ?? type)
       // Drop it where the user is looking, but never on top of another node.
       const position = freePosition(
         definitions,
@@ -470,6 +470,9 @@ function FlowEditorInner({
         params: {},
         requires: [],
         provides: [],
+        // A shared node brings the code; the ports and settings are this
+        // flow's own.
+        ...(sourceRef ? { source_ref: sourceRef } : {}),
       }
       const nextDefinitions = [...definitions, node]
       const nextCanvas = [
@@ -809,6 +812,9 @@ function FlowEditorInner({
         onSaveSource={(code) => {
           if (selected) sourceMutation.mutate({ nodeId: selected.id, code })
         }}
+        // Sharing rewrites the stored document, so the canvas takes the
+        // server's copy rather than keeping its own.
+        onShared={onReload}
         onClose={() => {
           flush()
           setEditorExpanded(false)
@@ -830,6 +836,7 @@ function FlowEditorInner({
         nodeTypes={nodeTypeInfo ?? []}
         flows={flows.data}
         onAddNode={addNode}
+        onAddSharedNode={(libName) => addNode("python", libName)}
         onRun={() => runMutation.mutate()}
       />